This invention relates to flexographic printing. More specifically this invention relates to an apparatus and method for correcting or preventing the misalignment of a flexible substrate during flexographic printing.
Flexographic printing, also known as flexography, was developed primarily for printing flexible substrate packaging materials such as corrugated boxes, displays or inserts. As more and more products are being packaged, manufacturers are using flexography to meet their packaging and labeling requirements.
Flexography is a method of direct rotary printing that uses resilient relief image plates of rubber or photo polymer material. The plates are affixed to plate cylinders of various repeat lengths, inked by a cell-structured ink metering roll, with or without a doctor blade, and carry a fast drying fluid ink to plates that print onto virtually any substrate, absorbent or nonabsorbent. For every revolution of the printing plate cylinder, an image is produced.
The flexographic printing process uses an inking press system, a typical example of which is shown in FIG. 1. Briefly, an ink reservoir 12 supplies ink 14 to a rubber fountain roll 16 which in turn supplies ink 14 to a metering roll 18. Metering roll 18 is often an anilox roll which has cells mechanically or laser engraved into the face of the roll. Anilox ink metering roll 18 applies a measured amount of ink 14 to the rotating printing plates 22 on the cylinder 24. The amount of ink 14 delivered to the plates 22 is metered by the screen size of the cells on the anilox roll.
In some systems on the surface of metering roll 18 is a reverse-angle doctor blade 20. If used, doctor blade 20 shears the ink 14 from the surface of the ink metering (anilox) roll 18 and transfers uniform levels of ink from cells in anilox roll 18 to the surface of printing plates 22 mounted to a printing plate cylinder 24. The system can also run without a doctor blade, and is then known as a two-roll flexographic inking system. In a two-roll system, ink fountain roll 16 is geared to run slower than metering roll 18 so that a wiping or roll-doctoring action takes place between rollers 16 and 18.
Flexographic printing plates 22 can be made of vulcanized rubber or a variety of ultraviolet-sensitive polymer resins. The plates 22 have a raised image and print directly to a flexible substrate 30 with a very light xe2x80x9ckissxe2x80x9d impression. Flexographic plates 22 are resilient and displaceable. Plates 22 are generally attached or mounted to plate cylinders 24.
Plates 22 carry ink 14 to a flexible substrate 30 as it travels through the ink press system. An impression roll 26 supports flexible substrate 30 at the point ink is being applied. Flexography uses fluid inks that are traditionally of low viscosity, highly fluid and quick drying. The inks are made into a dispersion of resins, solvents, color and additives which are either solvent or water reducible. These dry very quickly between the print stations of a press.
Flexographic printing presses either print on a continuous web of printable material or are equipped with a sheeter that delivers sheets instead of wound rolls. For high-quality images, the smoother the substrate the better.
The vast number of substrates on which flexography can print is one of its greatest advantages. There are a number of other advantages of flexographic printing. It can print on a wide variety of absorbent and nonabsorbent substrates. It uses fast-drying inks. It can print wet ink over dry ink to eliminate trapping problems, back-trap contamination and set off. It uses resilient rubber or photo polymer image carriers that can print millions of impressions. Presses can accommodate a wide range of cylinder repeat lengths to match print length requirements. Flexography is a near total variable repeat length system. Press speeds of 2,000 feet per minute or more are possible.
The printing plate cylinders can be removed from the press and plates can be mounted on individual plate cylinders held in a mounting machine. In a pre-production operation, proofs of flexible substrate 30 can be pulled from each cylinder to verify color-to-color registration along with any other specifications that need to be checked before running the job.
Ink on a printing plate may not be uniformly distributed. The image to be printed may need more ink on one side or the other, or there may be a need for more ink at the top or bottom with relatively less ink in the mid-section of the plate. This difference in ink distribution causes the friction between the printing plate and the flexible substrate to be not uniform. Differences in friction between different locations of a flexible substrate may cause the flexible substrate to pull or shift. This causes a deterioration in the quality of the printed image.
To correct for pulling or shifting during ink transfer, pull bands have been used horizontally on the leading edge or the trail edge of premounted printing plates and also along the side edges to correct the pulling or shifting of flexible substrate by presenting a uniform surface friction so that the machine can better grab or grip the sheet as it is pulled between the printing plate cylinder and the impression roll of the machine.
Some flexible substrates, such as corrugated sheets, often do not remain flat as they travel through a press. They may curl as they pass between the printing plate cylinder and the impression roll. To correct for curling edges, pull bands have been used horizontally onto either the leading edge or the trail edge of premounted printing plates to correct the warping of flexible substrate by pressing down on the starting or leading edge of the flexible substrate as it enters the press feed rollers, so that the machine can better grab or grip the sheet as it is pulled into the feed end of the machine.
The pull bands may be attached, such as by mechanically attaching them, to the sides of printing plates or, alternatively, are connected to the pull rolls of a printing press or die cutter.
Pull bands have been made as hand cut pieces of pre-manufactured rubber stock or have been manufactured by use of a standard liquid or sheet polymer plate making process. The hand cut pull bands could be made to different lengths to accommodate flexible substrates of varying sizes, but required substantial skill in addition to time and effort. Manufactured pull bands have been made in a limited number of discrete lengths. These manufactured pull bands often need to be cut shorter, resulting in waste. A multiplicity of pull band sizes must be purchased and stored, resulting in higher cost and storage needs.
Furthermore, problems such as the slipping or twisting of sheets, especially corrugated sheets, still exists, even when using the prior art pull bands. The prior art pull bands often do not keep sheets flat as they are run through the press.
Thus there exists a need for an apparatus and method which may be used to keep flexible substrates of varying sizes aligned with a flexographic printing plate for proper ink transfer.
It is an object of the present invention to provide an apparatus and method which keeps flexible substrates aligned as they travel through a press or die cutter.
It is a further object of the present invention to provide an apparatus and method which keeps flexible substrates aligned as they travel through a press or die cutter and which can be added to one or two sides of a premounted set of printing plates.
It is a further object of the present invention to provide an apparatus and method which keeps flexible substrates aligned as they travel through a press or die cutter and which can be added directly onto the pull rolls of a printing press or die cutter.
It is a further object of the present invention to provide an apparatus and method which keeps flexible substrates aligned as they travel through a press or die cutter and which may be used horizontally on the leading edge and/or the trailing edge of premounted printing plates to correct the warping of flexible substrates.
It is a further object of the present invention to provide an apparatus and method which can be used to keep flexible substrates aligned and can be used for varying sizes of flexible substrates.
The foregoing objects are accomplished in a preferred embodiment of the invention by an apparatus and method for pull bands which are connected to the printing plates or alternatively are connected to the pull rolls of a printing press or die cutter.
Further objects of the present invention will be made apparent in the following Best Mode For Carrying Out Invention and the appended claims.